Process for manufacturing powder-filled welded tubes, such as flux-cored welding wires

ABSTRACT

Process for manufacturing a welded tube containing filling elements, in which a long thin metal sheet is supplied continuously and formed into a gutter shape, into which the filling elements are introduced. Next, the sheet thus filled is made substantially into the form of a tube by bringing its two longitudinal edges close together until they are in contact or almost in contact with each other. According to the invention, the tube undergoes an axial rotation through an angle of between 45° and 110° to the vertical and the two longitudinal edges of the tube are welded together by a laser beam. Application of the process to the manufacture of flux-cored welding wire and the flux-cored wire thus obtained.

FIELD OF THE INVENTION

The present invention relates to a process for the continuous manufacture of tubes which are prefilled with filling elements, in particular, with powdery or granular materials, and then welded and possibly rolled and/or wire-drawn until their use diameter, particularly tubes intended to form flux-cored arc welding wires.

BACKGROUND OF THE INVENTION

Currently, to manufacture sealed flux-cored wires that can be used in arc welding, two processes are customarily used.

According to a first known process, a tube is continuously produced on a former, on leaving which the tube undergoes high-frequency (HF) welding. This welded tube is made in the form of a ring and then filled by being vibrated, with filling elements, such as powders and/or granules. Granulation is recommended for maintaining homogeneity of the powdery elements while the tube is being filled.

However, this filling phase is a lengthy and tricky phase of the process; and it determines the homogeneity, and therefore the quality, of the final product. If this filling phase is not completely under control, or a problem arises during this phase, the product obtained is impaired thereby.

According to a second known process, a tube is continuously produced on a former, and powdery and/or granular filling elements are introduced into a pre-tube, before the longitudinal edges of the tube are welded together, for example, by HF welding, arc welding, laser welding, or the like.

However, although HF welding is generally very suitable for ferromagnetic materials, it is found in practice that when the tube also contains completely nonmagnetic powdery elements, these are “sucked out” under the effect of the very intense magnetic field created by the HF welding current and contaminate the tube weld during production, resulting in defects or, at the very least, increased brittleness of the weld seam which cannot withstand subsequent conversion operations that it has to undergo, such as the usual wire-drawing and rolling steps, without fracturing.

To try to overcome the abovementioned drawbacks and problems occurring in HF welding, when the tube contains magnetic powdery elements, it has been proposed to carry out arc welding of the longitudinal edges of the tube by, for example, a multi-electrode TIG process or by a laser beam.

However, these processes result in other problems or limitations.

Thus, multi-electrode TIG welding is a relatively slow process if it is desired to have full penetration during welding. Thus, a welding speed of barely 3.5 m/min is obtained for a weld thickness of 2.2 mm, despite the simultaneous use of eight TIG electrodes aligned along the weld seam plane to be produced. Of course, such a speed is completely insufficient from an industrial standpoint.

Laser welding, owing to the high energy density that it generates, does allow full penetration to be obtained when welding the tube, and with welding speeds of three to four times higher than those obtained with a multi-electrode TIG welding process.

However, to obtain full penetration with a laser beam, it is necessary for the laser beam, which impinges on the weld perpendicularly, to emerge inside the tube.

It will be immediately understood that this affects the powder inside the tube, that is to say that lying along the path of the laser beam, since the laser beam will necessarily enter the tube and strike the surface of the filling elements contained in the tube, damaging them, since the beam arrives vertically on said filling elements. This process is, therefore, unsuitable for welding tubes prefilled with filling elements before they are welded, and it can be used effectively only with empty tubes.

Now, introducing the filling elements before the tube is welded, for example, by a belt system, has the advantages of eliminating the necessary granulation when filling the tube after welding, as explained above, and results in very good homogeneity of the powder blend and to constancy of the fill factor, which are necessary conditions for achieving acceptable quality of the finished product, in particular when the welded tube filled with powders is intended to be used as flux-cored welding wire.

Such processes are, for example, described in U.S. Pat. No. 5,192,016, EP-A-812 648, EP-A-489 167 and EP-A-589 470.

The problem that arises is therefore how to provide an improved process for the continuous industrial manufacture of a tube containing filling elements, in particular powders or the like, which tube is closed off by welding along its two longitudinal edges after it has been filled with the filling elements, which process makes it possible to carry out full-penetration welding over the entire thickness of said tube at speeds of the order of those obtained in conventional laser welding, but without causing the abovementioned problems of damaging the filling elements contained in the tube.

SUMMARY OF THE INVENTION

The solution is a process for manufacturing a welded metal tube containing filling elements, in which the following successive steps are carried out:

a long narrow metal sheet having two longitudinal edges is continuously supplied;

at least part of said long narrow metal sheet is made in the form of a gutter by bringing one of its two longitudinal edges close to the other;

filling elements are introduced into the gutter-shaped metal sheet;

at least one part of said long narrow metal sheet filled at step (c) is made substantially in the form of a tube by continuing to bring one of its two longitudinal edges closer to the other until said two longitudinal edges are in contact or almost in contact with each other,

wherein after step (d):

the tube containing the filling elements is made to undergo an axial rotation through an angle between 45° and 110° to the vertical; and

said two longitudinal edges of the tube that have been brought into contact with each other in step (d) are welded by a laser beam.

Depending on the case, the process of the invention may include one or more of the following features:

the tube undergoes full-penetration or almost full-penetration welding;

in step (e), the tube undergoes an axial rotation through an angle between 60° and 105° to the vertical;

in step (e), the tube undergoes an axial rotation through an angle between 80° and 100° to the vertical;

in step (e), the tube undergoes an axial rotation through an angle between 85° and 95° to the vertical;

in step (e), the tube undergoes an axial rotation through an angle of around 90° to the vertical;

during steps (a) to (f), the metal sheet is furthermore made to undergo a continuous translational displacement movement, for example, by means of motorized rotary drive rollers;

the filling elements occupy up to 50% of the internal volume of the tube;

after step (f), the welded tube obtained is wire-drawn and/or rolled;

the welded tube obtained is a flux-cored arc welding wire; and

the sheath of the tube is made of steel; however, the invention is not limited to the welding of steel sheet and may be applied to any ferrous or non-ferrous weldable metal, for example, aluminum or its alloys.

The invention also relates to a flux-cored arc welding wire formed from an external metal sheath, in particular made of steel, containing powdery and/or granular filling elements, said external sheath having a full-penetration longitudinal weld seam, obtained directly by the process according to the invention.

Preferably, less than 2%, more preferably less than 1%, and even more preferably less than 0.6%, of the filling elements that it contains are damaged during the welding of said tube.

Advantageously, it contains rutile as filling element.

In other words, the present invention proposes to improve the continuous processes for manufacturing tubes, especially flux-cored welding wires, in which the filling of the tubes with powdery or granular powders, for example, using metering conveyors or the like, takes place before the longitudinal edges of the metal sheet used to produce the external sheath of the tube are welded together and in which the welding is then carried out by means of a laser beam with full penetration without affecting or damaging the powders introduced thereinto beforehand.

To do this, a metal sheet made, for example, of steel, having two longitudinal edges parallel to each other is firstly made, in a manner known per se, in the form of a gutter, that is to say having substantially a U shape (in cross section), by mechanically deforming it using mechanical deformation means, such as press rolls and forming rolls, so as to bring the two longitudinal edges close together.

The metal sheet is also made to undergo a translational movement in the direction of its longitudinal axis so as to have a continuous manufacturing process. This means that the mechanical deformation and the forming of the sheet take place progressively as it advances, that is to say as it is driven by drive means, for example, motorized rotary drive rollers.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages will become apparent in view of the appended figures, in which:

FIG. 1 schematically shows an embodiment with a tube in the form of a U; and

FIG. 2 is a cross-sectional view of a welded wire obtained by a process of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Once the gutter or tube 1 has been made in the form of a U, as illustrated schematically in FIG. 1, it is filled (arrow 10) with the filling elements 2, such as metal powders or the like, via an opening 3, which is vertical, i.e., located at the top of the tube 1.

Next, as may be seen in FIG. 1, the tube 1 is pivoted by another set of rollers through a predefined angle, preferably an angle of rotation R of around 90°, about its longitudinal axis so as to be able to weld, by horizontal-position welding using a laser beam 5, the two longitudinal edges 4 together, and no longer vertically as in the prior art. In other words, according to the invention, the laser beam arrives at the tube laterally and no longer vertically, as practised in the prior art.

The upper level of powder contained in the wire 1 is constrained to remain in a horizontal plane by a vibrating system. This vibration does not cause longitudinal segregation and, therefore, does not impair the homogeneity of the product.

The edges of the tube 1 are then brought closer together until they are in contact or almost in contact with each other, that is, so as to leave a very small space, or even no space, between the two edges, thus forming a weld seam plane. This action of bringing the edges together may be accomplished, for example, by press rollers or by any other means for mechanically deforming the metal sheet so as to give it an O-shape (seen in cross section), that is, a circular, oval, elliptical, or similar shape. However, other shapes are also possible.

Due to the density of the powders and the fill factors usually employed, care is taken to ensure that the upper level of the powder lies below the horizontal mid-plane of the tube 1 so that the beam 5, although emerging therein, does not affect the powder 2 during full-penetration welding.

In other words, as shown in FIG. 1, the quantity of filling powder 2 introduced into the tube 1 when filling it is chosen so that its maximum level or high level remains below the weld seam plane formed by the edges 4 of the tube.

Thanks to the process of the invention, the presence of the powder 2 in the tube 1 no longer precludes it being welded by full-penetration welding using a laser beam 5, which is essential for withstanding the stresses during the subsequent rolling operations, thanks to the rotation R of the tube 1 through a predetermined angle, preferably around 90°.

Furthermore, the process of the invention also has the advantage of making it considerably easier to set the parameters of the laser beam, as it is no longer necessary, as in the prior art, to set the beam at “the limit of emergent penetration”, which is difficult to control and is the source of blisters in the weld. Consequently, the process of the invention is much more reliable from the industrial standpoint.

The process of the invention has been applied to the full-penetration welding, in a horizontal position, as described above and illustrated in FIG. 1, of a steel tube approximately 2.2 mm in thickness with a speed of 11 m/min, said tube containing a rutile (TiO₂) powder with a fill factor of 18.6%.

After welding, the welded tube thus obtained was analysed and found that the content of powder “licked” by the laser beam was barely 0.4%. For comparison, tests carried out with a process according to the prior art, that is to say with identical welding but in a vertical position and with penetration at the “emergent limit”, gave a content of degraded powder of 4%.

It follows that the process of the invention results in 10 times less degradation of the filling powder and also makes it possible to use a powder of small particle size.

Moreover, the quality of the weld seam obtained, during full-penetration welding of the tube, is excellent, as witnessed in FIG. 2, which is a cross-sectional view, at the weld seam obtained, of the welded wire obtained by the process of the invention.

The tube manufacturing process according to the invention, therefore, makes it possible to produce flux-cored welding wires filled with filling elements before welding and then welded by full-penetration laser welding, without impairing the filling elements that they contain, particularly filling powders of small particle size. 

1. Process for manufacturing a welded metal tube containing filling elements, in which the following successive steps are carried out: a) a long narrow metal sheet having two longitudinal edges is continuously supplied; b) at least one part of said long narrow metal sheet is made in the form of a gutter by bringing one of its two longitudinal edges close to the other; c) filling elements are introduced into the gutter-shaped metal sheet; d) at least one part of said long narrow metal sheet filled at step c) is made substantially in the form of a tube by (continuing to bring one of its two longitudinal edges closer to the other until said two longitudinal edges are in contact or almost in contact with each other, characterized in that, after step d): e) the tube containing the filling elements is made to undergo an axial rotation through an angle between 45° and 110° to the vertical; and f) said two longitudinal edges of the tube that have been brought into contact with each other in step d) are welded by a laser beam.
 2. Process according to claim 1, characterized in that the tube undergoes full-penetration or almost full-penetration welding.
 3. Process according to claim 1, characterized in that, in step e) the tube undergoes an axial rotation through an angle between 60° and 105° to the vertical.
 4. Process according to claim 1, characterized in that, in step e), the tube undergoes an axial rotation through an angle between 80° and 100° to the vertical.
 5. Process according to claim 1, characterized in that, in step e), the tube undergoes an axial rotation through an angle between 85° and 95° to the vertical.
 6. Process according to claim 1, characterized in that, in step e), the tube undergoes an axial rotation through an angle of around 90° to the vertical.
 7. Process according to claim 1, characterized in that, during steps a) to f), the metal sheet is furthermore made to undergo a continuous translational displacement movement, for example by means of motorized rotary drive rollers.
 8. Process according to claim 1, characterized in that the filling elements occupy up to 50% of the internal volume of the tube.
 9. Process according to claim 1, characterized in that, after step f), the welded tube obtained is wire-drawn and/or rolled.
 10. Process according to claim 1, characterized in that the welded tube obtained is a flux-cored arc welding wire.
 11. Flux-cored arc welding wire formed from an external metal sheath containing powdery and/or granular filling elements, said external sheath having a full-penetration longitudinal weld seam, characterized in that it is obtained directly by the process according to claim
 1. 12. Flux-cored wire according to claim 11, characterized in that less than 2% of the filling elements that it contains are damaged during the welding of said tube.
 13. Flux-cored wire according to claim 11, characterized in that less than 1% of the filling elements that it contains are damaged during welding of said tube.
 14. Flux-cored wire according to claim 11, characterized in that less than 0.6% of the filling elements that it contains are damaged during the welding of said tube.
 15. Flux-cored wire according to claim 11, characterized in that it comprises a steel sheath and/or it contains rutile as filling element.
 16. Flux-cored wire according to claim 12, characterized in that less than 1% of the filling elements that it contains are damaged during welding of said tube.
 17. Flux-cored wire according to claim 12, characterized in that less than 0.6% of the filling elements that it contains are damaged during the welding of said tube.
 18. Flux-cored wire according to claim 12, characterized in that it comprises a steel sheath and/or it contains rutile as filling element.
 19. Process according to claim 2, characterized in that, in step e) the tube undergoes an axial rotation through an angle between 60° and 105° to the vertical.
 20. Process according to claim 2, characterized in that, in step e), the tube undergoes an axial rotation through an angle between 80° and 100° to the vertical. 